Hoyas, various species within the genus of the same name, are perennials prized for their vining habit, waxy leaves and attractive flowers. Most, like the wax plant or wax flower (Hoya carnosa), are grown primarily as houseplants as they can only survive outdoors in U.S. Department of Agriculture plant hardiness zones 10 to 11 and are injured by temperatures below 40 degrees Fahrenheit. Hoyas tend to have a relatively small root system and perform well when root growth is slightly restricted. However, when soil appears dry shortly after watering or the plant's roots are growing out of the container's drain holes or above the soil surface, the hoya will benefit from being transplanted into a slightly larger container.
Clean an appropriate container to transplant the hoya into if the container was previously used. Select a container just 1 to 2 inches larger than the plant's current container. Remove any large dirt clods or other debris that might still be in there, and soak the container in a 10 percent bleach solution or clean it using a household disinfectant. Rinse it with clear water.
Water the hoya well several hours before you transplant it to make the hoya easier to remove from its current container and to minimize stress to the plant.
Place a small amount of well-drained potting soil in the bottom of the new container. Add just enough soil so that the point where the hoya stem will meet the soil line is about an inch below the lip of the container.
Slide the hoya's root mass out of its current container. Use one hand to hold the plant by the soil surface while you turn the plant on its side or upside down and remove the container with your other hand. You may need to tap the container's lip on the edge of a counter or other structure to loosen the root mass.
Inspect the hoya's root mass and use a sharp, sterile knife to trim off any dead, brown, mushy or damaged roots. If the hoya is severely root-bound, with roots growing in a tight circle around the edge of the root mass, use the sharp knife to score the root mass. Make four vertical, evenly-spaced cuts about 1 inch deep around the circumference of the root mass and cut an "x" into the bottom of the hoya's root mass.
Set the hoya's root mass in the center of the prepared container, adding or removing potting soil under the root mass, if needed, to adjust its height in the container.
Fill in the remaining space around the root mass with high-quality, well-drained potting soil. Firm the soil down gently to force out large air pockets, water the soil in slowly and thoroughly, and add more soil, if needed, to account for any settling.